The present invention relates to electrical power conversion. More particularly, the present invention relates to a power converter having dynamic current limiting.
Electrical power is supplied in one of two forms: direct current (DC) power and alternating current (AC) power. There are often times when it is desirable to convert one form of electrical power to another form. A power converter can convert power from AC to DC, DC to AC, AC to AC, or DC to DC. In this way, a power converter allows a device that uses one form or level of power to connect to a power source that supplies a different form or level of power.
Most power converters have a surge power rating, which is typically a multiple of a continuous rating. The surge power rating is typically about twice the continuous rating, and in some cases may be as high as three or four times the continuous rating. A power surge from a power converter is needed, for example, when an electrical motor first starts. There must be enough current delivered from the power converter to initiate rotation of the rotor of the motor. This involves overcoming static and inertial forces in the motor. As a result, a higher level of current is required for a short period of time when first starting an electrical motor. Once the motor is turning, the power demand is reduced to the normal operating range of the converter (i.e. below the continuous power rating).
One advantageous form of power converter makes use of a high frequency transformer in conjunction with an input circuit which produces high frequency pulse width modulated current pulses to the primary of the transformer. An output circuit connected to the high frequency transformer converts the transformed pulse width modulated pulses into the desired form of output power, such as continuous wave AC power. Power converters of this type generally include some form of feedback control which will limit maximum current to a level corresponding to the surge power rating. The feedback circuit typically senses voltage at the output of the power converter. When output voltage decreases (such as when a motor is being started), the feedback circuit causes the normal current limit of the input circuit to increase up to the maximum or surge limit. Current is allowed to remain at the higher level for a set time period (for example: five seconds). If the current does not decrease by the end of that time period, the power converter stops so that the higher current levels do not damage electrical components of the converter or the load (such as a motor) connected to the output of the power converter.
This type of current limiting feedback control causes a delay in matching the maximum current limit to the demand for higher current. It is immediately when the motor turns on that the highest current is needed, yet the feedback control requires numerous AC cycles to increase the current limit up to the surge power rating. In the meantime, the increasing current causes electrical components such as transistors within the power converter to heat up, and causes increased heat within the motor, without providing enough current to break the motor free and allow it to start turning. Ideally, the current limit should be at its highest when the motor first calls for current and then should decrease over time. Prior current limiting feedback control, however, does not allow current to be at a maximum when current is first called for by the motor, but rather introduces a time delay before maximum current is available.